1. Field of the Invention
The present invention relates to a rotary electric machine such as a stepper motor, which includes a stator including a plurality of main poles with windings arranged therearound and a rotor arranged inside the stator to be rotatable with respect to the stator.
2. Description of the Related Art
Stepping motors have been used as driving portions in a wide variety of areas including information equipment such as printers, facsimile machines, and copiers and industrial equipment such as FA instruments. The overall demand for stepper motors is growing every year and the applications of stepper motors are also expanding. In addition, demands for size reduction and cost reduction of stepper motors, which can contribute to the overall cost of equipment, are also increasing. Especially, the rotary electric machines such as stepper motors frequently used in OA instruments including copiers and facsimile machines are required to reduce the size and the cost, increase the rotation speed, provide a higher torque, and rotate with lower vibration.
In a case of a conventional hybrid type stepper motor using a magnetic material portion and a permanent magnet in a rotor, the structure disclosed in U.S. Pat. Nos. 5,283,486 and 5,410,200, for example, is frequently used. In these structures, a stator has a loop-like magnetic material frame with a plurality of main poles radially projecting inward and windings arranged around the respective main poles. The stator is arranged to oppose the hybrid type rotor in which the axially magnetized permanent magnet is interposed between a pair of magnetic material portions with an air gap.
Among the above described stepper motors, one having a step angle of 1.8° is frequently used. To keep and control that step angle with high precision, it is necessary to control the air gap between the stator and the rotor to 0.05 mm, for example. Thus, in the typical stepping motor having the step angle of 1.8°, a bush made of metal such as aluminum is fixed to a motor case supporting the stator and is arranged to hold a ball bearing, and the rotor shaft of the rotor is rotatably supported by the ball bearing with high precision.
However, in the above structure of the stepper motor, it is necessary to use the metal bush and the ball bearing in order to ensure a minute air gap between the stator and the rotor. This may increase the cost of the motor.